The first lesson in demeanor evidence is its long-standing legal tradition. The Melendez-Diaz case did not arise from nowhere. Western justice has long used demeanor evidence to prove guilt, albeit not always in ways we would now approve. In 1050 c.e. Lady Emma, twice crowned Queen of England, was accused of adultery with the Archbishop of Canterbury. To test her deception or innocence, she had to walk barefoot over nine red-hot plowshares. If her skin was undamaged, she was innocent. But if her feet were scalded, she would be hung. Her son, the King, was already busy claiming her properties and rescinding her titles. To everyone’s amazement, however, she walked across the plowshares unharmed and her repentant son begged for forgiveness.6 At least that’s what legend says.
Thankfully, today’s courts value demeanor even more, but apply it differently. The U.S. Supreme Court in U.S v. Scheffer7 (1997) said that juries are “lie detectors” and another Supreme Court ruled that credibility in witness assessments includes tone of voice and variations in behavior.8 Jurors use witness demeanor as human technology, not to detect lies in forensic experts, but to determine both scientific truth and persuasive truth.
New Insights from Legal Research and Neuroscience
Both neuroscience and juror research reveal helpful insight for expert witnesses and this article will apply them to forensic scientists. First, telling scientific truth and testifying to persuasive truth is not the same thing. Being technically correct is not the same as being persuasively competent. Forensic scientists must distinguish the credibility of their own testimony from the conclusion of their opposite number. After all, qualified forensic scientists on both sides will testify to opposing scientific truth. Juror research teaches that demeanor credibility consistently ranks as a significant factor in juror persuasion.
Second, demeanor evidence is important because it is how humans communicate and make decisions. One of the lessons of neuroscience is that our demeanor says much more than our words. Scholars estimate, for example, that at least 80% of our communication is nonverbal.9 Everyone notices the processes of communication more quickly and more efficiently than the content of communication. Jurors generally find how a forensic expert conveys their research equally compelling as the results themselves. Words conveying information came later in our evolutionary stage than physical expression. Our prehistoric ancestors had to quickly scan the environment for predators. In fact, a large part of our demeanor assessments are conducted in the brain’s most ancient regions.10 So, demeanor evidence is an instinctual part of our behavior.
Yet, our demeanor assessments are not always correct. Evidence shows that judges and juries not only rely upon demeanor evidence in assessing witness credibility, they report that they are pretty good at it.11 Jurors will depend more upon non-verbal cues to assess credibility as expert credentials and forensic technology become more complex and standardized. Forensic scientists will likely face increasingly sophisticated demeanor testimony as more and more scientists are called to testify. They serve justice by presenting competent demeanor evidence.
Third, not only do judges and jurors scrutinize witness demeanor, but they do so almost instantaneously. Research has shown that our brains process demeanor as quickly as 100 milliseconds. 12 This means that jurors size up experts before they utter their first word or even take the stand. Jurors will begin assessing the forensic scientist even as he or she walks into the courtroom or takes the oath. Those immediate impressions tend to last throughout deliberations.
Fourth, research also shows that juries find the eyes of a witness to be the single most important visual cue to assess credibility.13 In one case, a defendant insisted upon wearing sunglasses at the trial and unsuccessfully appealed the conviction based upon how the sunglasses may have undermined the testimony.14